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Integrated Organic Rankine Cycle system for simultaneous utilization of solar energy and LNG cold energy

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Solar-driven generation system recovers cold energy for electricity and heat

EU-funded researchers have developed an organic Rankine cycle that generates electrical and thermal energy from different types of heat resources: concentrated solar systems and cold energy contained in liquefied natural gas.

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Rising electricity demand, finite fossil fuel reserves and environmental concerns have motivated the switch to renewable energy sources. But one of the biggest problems that renewable energy technologies, such as solar, pose is that energy is generated while the Sun is shining. Night-time and overcast can interrupt the supply. Furthermore, solar radiant energy has a low energy density and, therefore, its practical utilisation requires large-area solar collectors.

Integrated system resolving intermittency and efficiency issues

Parabolic troughs, a type of solar thermal collectors, are now one of the lowest-cost devices to produce power from the Sun. They can effectively deliver useful heat at temperatures between 50 and 400 degrees Celsius. This thermal energy can be converted into power using one of the most versatile and efficient power cycles: the organic Rankine cycle (ORC). This closed thermodynamic cycle together with solar parabolic troughs and thermal energy storage systems provides the flexibility needed to deal with the intermittent character of solar energy. “In the context of the SO-LNG-ORC project, we investigated the optimal design and operation of an ORC system combined with solar thermal collectors and sensible heat storage to overcome solar energy intermittency,” notes Haoshui Yu, a researcher involved in SO-LNG-ORC. The project was funded by the Marie Skłodowska-Curie Actions programme. “We developed an integrated ORC system that utilises both solar energy and cold energy stored in liquefied natural gas (LNG). Cryogenic heat sinks like LNG can further improve ORC efficiency by utilising the cold energy, which is otherwise wasted during regasification,” explains Yu.

Streamlining system performance through simulations

Researchers carried out simulations to determine the optimal design of the two-tank thermal energy storage system and the most suitable temperatures of the hot and cold storage tanks. Furthermore, they investigated the optimal mass flow rates of the heat transfer fluid, which determine both the temperature of the circulating fluid and the heat absorbed by the solar thermal collector. Tests were also conducted to identify the most suitable heat transfer fluids for cold energy recovery from LNG. “When the ORC system utilises solar energy and LNG cold energy simultaneously, the optimal heat transfer fluid may differ. System configuration may also become more complicated, involving cascade or parallel cycles,” remarks Yu. The ORC system driven by solar energy generated round-the-clock stable power output. “Using simulation-based optimisation, the system efficiency of the proposed ORC power plant using toluene as a heat transfer fluid increased from 17.9 % to 24.8 %,” notes Yu. ORC recuperating waste heat (cold energy) demonstrated improved performance compared to that of the standard ORC. “We were the first to explore the synergy between an ORC system with solar energy and cold energy stored in LNG,” highlights Yu. “Project results will pave the way for energy systems that are highly efficient and profitable yet present little risk and have a low environmental impact.” The proposed system is described in the article ‘Optimal design and operation of an Organic Rankine Cycle (ORC) system driven by solar energy with sensible thermal energy storage’, published in the peer-reviewed scientific journal Energy Conversion and Management. Furthermore, the 2021 conference paper titled ‘Performance comparison of organic Rankine cycle (ORC) and CO2 cycle for simultaneous utilization of liquefied natural gas (LNG) cold energy and solar energy’, compared the ORC and CO2 cycle performances utilising LNG cold energy and solar energy.

Keywords

SO-LNG-ORC, LNG, cold energy, solar energy, heat transfer fluid, liquefied natural gas, electricity, intermittency, organic Rankine cycle

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